Circuit boards and circuit cards find a wide variety of uses in the electronics industry.
A widely employed technique for preparing such is to impregnate a woven fiberglass sheet with a resin composition and then laminate a copper sheet to one or both sides of the resin impregnated fiberglass sheet. Next, an electrical circuit is etched into the copper to form the circuit board or card and then electrical connections can be soldered to it when it is used.
Various resins have been suggested for the purpose of impregnating the fiberglass.
For example, epoxy resin laminated impregnated glass fabrics are widely used in the preparation of printed circuit boards and cards. The epoxy compositions for such purpose should possess a number of important characteristics. For example, such are required to be solvent resistant since they are repeatedly exposed to processing solvents and their vapors. For example, photoresists are stripped from the surface of metal and bonded to the surface of the laminate by exposing the photoresist to halogenated hydrocarbons.
Resin composition, when cured, should desirably exhibit a relatively high glass transition temperature and possess high resistance to elevated temperatures. Accordingly, compositions to be suitable as the matrix material for integrated circuit boards must possess a number of diverse characteristics including relatively high glass transition temperature when cured, solubility and stability in low boiling point solvents, adhesion to the fiberglass sheets, low thermal expansion, and high electrical resistivity.
Furthermore, the laminates should also be self-extinguishing (i.e., the epoxy impregnant should be a fire retardant).
A number of epoxy-containing compositions have been suggested for such purposes and include those compositions disclosed in U.S. Pat. Nos. 3,523,037; 4,294,877; 4,294,743; and 4,550,128, disclosures of which are incorporated herein by reference.
In particular, epoxy compositions along the lines of the disclosure of U.S. Pat. No. 3,523,037 has been used in the preparation of power supply modules. Typically, power supply modules consist of a number of components surface mounted onto a circuitized card. The card consists of a thick copper plate (either 20 or 40 mil), a thin layer of the epoxy-fiberglass dielectric (one or two plies of prepreg), and three ounce copper circuitry.
Although these cards or boards are satisfactory for the most part, the thermal conductance of the dielectric could stand improvement. In particular, the ability of this type of card to transfer heat away from the components which have maximum allowable operating temperature of 110.degree. C. is very limited by the thermal conductance of the dielectric (the conductance of copper is very high). Therefore, increase in the thermal conductance of the dielectric layer would be desirable in order to extend current power supply packaging to higher power and higher thermal flux (increased power, constant card area) applications.